The use of concrete as a building material is widely known as is its fundamental strength is in compression and its weakness is in tension. It is very desirable in many construction applications to utilize materials which can withstand both compressive and tensile forces. As concrete is typically unable to resist tensile forces, some type of tensile reinforcement must be utilized with the concrete.
Pre-stressed concrete utilizes reinforcement by high strength steel which is pre-stressed within the concrete thereby providing active tensile reinforcement within the concrete versus the passive reinforcement which resulted with the traditional, rebar-reinforced concrete. Such active reinforcement has been found to dramatically extend the range of applications where concrete can be used.
In a typical tendon tensioning anchor assembly used in post-tensioning operations, a pair of anchors is used for anchoring the ends of the tendons suspended there between. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, a wedge, threaded nuts or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as water, de-icing chemicals, sea water, salt water, brackish water, or spray from these sources. Wire cable corrosion is a significant concern in post tension systems. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. Also, tendon failure can occur due to water intrusion into the interstices between the tendon and is typically concentrated at tendon ends or anchors. This can cause a premature failure of the post-tensioning system and a deterioration of the structure.
Tendon failure can occurs at portions of the tendon remote from the anchor if it is damaged during installation. The installation of tendons typically occurs in a rugged construction environment where the tendons can be damaged by equipment, careless handling and contact with various site hazards. When the elastomeric sheath is punctured, a water leak path contacting the wire tendon is established. The puncture must be patched to resist water intrusion between the sheath and tendon.
Tendon corrosion typically occurs near the post-tension anchors because the outer sheath is removed from the wire tendon at such locations. To protect the bare wire from corrosion, protective tubes are connected to the anchor and are filled with grease or other corrosion preventative material. This conventional practice is demonstrated by different post-tension systems. Some conventional approaches attempt to create a water tight seal between portions of an encapsulated anchor and the tendon, such as shown in U.S. Pat. Nos. 5,749,185; 6,023,894; and 6,883,280.
Unfortunately, these conventional systems do not prevent water intrusion in all circumstances due to tendons and their sheathing lacking dimensional integrity. Tendons can come from a wide variety of manufactures with large tolerances in outside diameter of the tendon and its protective sheath. Due to the wide variety of tendon dimensions for a nominal size, conventional seal arrangements designed to fit the largest diameter tendons, lack sufficient sealing on lowest diameter tendons of the same nominal thickness. Additional factors potentially causing seal problems include shrinkage and/or other dimensional changes of the sheath, encapsulation, sealing materials, or any combination thereof.
A need exists for an improved post-tension system which better resists corrosion than conventional technology. The system should be compatible with existing installation procedures and should resist the risk of water intrusion into contact with internal tendon wires.